I also revisited Disgaea 3 today, since a recent patch added trophy support to it.
Unfortunately none of the trophies were awarded retroactively, but it turns out that I wouldn’t have gotten very many anyway. A lot of them are for fairly rare or tricky things, and I spent a while working on them. I mainly worked through all 100 floors of an item world while in the Land of Carnage, which got me a trophy for doing so, a bunch of the pirate trophies, and a couple for mystery floors seen along the way.
I have around 1/3rd of the trophies so far. I’m not sure how much time I’ll spend on them, since some of them require just plain luck in tough spots.